Terahertz imaging system using hot electron bolometer technology
Terahertz radiation (T-ray) can penetrate clothes, dust, and smoke better than infrared and visible light. T-rays' shorter wavelengths offer higher spatial resolution than microwaves or millimeter waves. This has opened the field for security applications such as remote sensing and detection of concealed weapons and illicit drugs. Furthermore, the ability of terahertz radiation to penetrate biological materials to some extent opens the field of imaging and spectroscopy for healthcare applications.
Current terahertz imaging systems have some limitations. We are developing a two-dimensional, passive, heterodyne imaging system centered at 850 GHz. For security and biomedical imaging applications, the detector elements should have very high sensitivities throughout the entire terahertz spectrum and low LO power consumption. These requirements exclude Schottky barrier diode mixers and SIS mixers, but can be fulfilled by NbN hot electron bolometric (HEB) mixers. The sensitivity of the imaging system can be improved by making the IF bandwidth of the receiver as wide as possible. In order to show the feasibility of this type of system, we have designed a compact receiver block consisting of an HEB mixer integrated with an MMIC low noise amplifier in close proximity. The HEB mixer is quasi-optically coupled by the use of an elliptical silicon lens and a twin-slot antenna tuned to a specific frequency of interest (850 GHz for the current system). A harmonic multiplier sources, providing an output power level of about 250 μW, is employed as the local oscillator.
The receiver, currently used in the imaging system, exhibits a noise temperature of about 1800 K, an effective bandwidth of 2.6 GHz, and an Allan time of 1.2 seconds. A thermal sensitivity of 1 K and a spatial resolution of about 8 mm have been demonstrated. Further statistical analysis shows a RMS temperature fluctuation of about 0.4 K, corresponding to a resolvable temperature difference of 1.2 K within a 99.7% confidence interval. Future development includes a new architecture for large focal plane arrays using multiple HEB detectors for real-video rate imaging applications.